Weighing platforms that can dynamically weigh food items while being conveyed is becoming a standard component in food processing systems. The food industry needs to be able to weigh individual pieces of food as well as batches weighing thousands of kilos. An example of batch weighing is when a fishing vessel is e.g. selling 100 tons fish to a processing factory. Inside the processing factory it is common to weigh individual fishes or pieces of fish that enter various food processing systems for different reasons such as to determine the total input weight, tracking, forming end user packages and so on.
It is convenient to use flow scales for weighing big batches of food. Flow scales add up every piece of food that is moved over them and display the result as total kilo or lbs. Flow scales are typically formed by placing a load sensing device with some form of platform under a belt that carries the items to be weighed. Such flow scales could become a standard component in food processing systems.
FIG. 1a-c depicts schematically the principle of a prior art flow scales for dynamically weighing items, e.g. food items. As depicted here, an in-feed conveyor 101 conveys a food item 104 to a flow scale 102 that comprises a weighing platform with an associated closed loop conveyor belt on the top that conveys the item over the weighing platform while registering weighing-related signal. A take-away conveyor 103 is situated adjacent to the flow scale for receiving and conveying the weighed food item for further processing. Since a single item is being weighed at a time, i.e. one item is located on the weighing platform 102 at a time, the distance between adjacent food items must be at least as much as the length of the platform 102.
Many of the food approved platforms are made of plastic material and are prone to deform, where this deformation becomes larger the larger the platform is. All such deviations from the original shape of the weighing platform will affect the weighing accuracy. Referring to FIG. 1a-c, height differences that may occur at the in-feed and out-feed ends between the weighing platform 102 and the conveyors 101, 103 will affect the weighing accuracy because the force on the load receiver 105 will change according to the torque needed to run the belt on top of the load receiver 105. Also, when the food item enters the weighing platform (see FIG. 1b) it is pushed somewhat downward as indicated by the arrows, as well as when the food item leaves the weighing platform to the take-away conveyor (see FIG. 1c), but such a tilting affects the weighing accuracy. It is of outmost importance that the belt that is used for such flow scales is very flexible in the vertical direction since otherwise the load sensor will not sense all the weight of the material and an error will be introduced.
Other type of flow scales that are frequently used in the industry for weighing continuous flow of items are flow scales such as the one shown in FIG. 2 comprising four wedge shaped rollers 301, two rollers acting as weighing rollers 304, 305 and two rollers acting as fixed rollers 302, 303 at the opposite ends, where rubber/vinyl belt (not shown) is typically used to hold the material that is being weighed. Such U-shaped flow scales are well known in industries where accuracy is not important and where large amount of material is being weighed over a relative long time. The U-shaped belt that is used is not flexible and to counteract that drawback it is common to increase the length of the weighing section to several meters and use more load sensing devices. For weighing platforms of this kind, it is of outmost importance that the rollers are at the same height and that the stiffness of the belt remains fixed at all times. Also, the stiffness of the belt varies with temperature fluctuations and may thus easily affect the measuring results.
GB 1 584 981 discloses a U-shaped flow scale similar as depicted in FIG. 2 but further comprising U-shaped continuous plate structures forming a support for the conveyor belt, where a pair of first supports for the belt are spaced apart in the direction of the length of the belt and at least one second support for the belt is positioned there between, where the second support is associated with weigh means. This apparatus is especially suitable for weighing uniform material such as sand, gravel etc. due to the U-shape surface of the supports (preventing it from flowing from the sides). The consequences of using supports having such U-shape is that carrying capacity is created in the belt lying on top on the U-shaped supports creating an upwardly extending vertical force, but this force affects the weighing accuracy greatly. Because of this, such weighing apparatuses having such U-shaped forms are relative long, typically several meters, so as to reduce the carrying capacity effect of the U-shaped belt, i.e. the longer the U-shaped belt is the less will the stiffness in the belt be and thus the less will the upwardly extending vertical force be. This U-shaped flow scales are thus very spacious.
Moreover, the carrying capacity in the belt due to the U-shape will change when external conditions such as the temperature change. An increase in the temperature will make the belt softer which consequently magnifies the weighing signal because the belt has more tendencies to lie on the U-shape support, i.e. the load cell, and vice versa, a decrease in the temperature will make the belt more stiff and thus increases the upwardly extending vertical force having the opposite effect on the weighing signal.
Also, any kinds of deformation in the supports due to change of external conditions such as thermal fluctuation such as twisting will affect the weighing accuracy but any misfits that are formed due to such deformation between the second and first supports cause that incoming load may either partly hit the second support or partly fall onto the second support, but such extra impacts cause an error in the weighing accuracy. Moreover, since the supports are solid structures external conditions such as draught may further affect the weighing accuracy of the weighing apparatus in GB 1 584 981.
The inventor of the present invention has appreciated that there is thus a need for an improved weighing platform that is capable of measuring a flow of items, where the above mentioned uncertainty in measuring is reduced and has in consequence devised the present invention.